1. Field of the Invention
The present invention relates to a positioning device for performing a retracting action, which is mounted to apparatuses such as a machine tool or a three-dimensional measuring device in consideration of safety.
2. Description of the Related Art
A machine tool and a three-dimensional measuring apparatus enable arbitrary positioning control, and may often perform driving at high speed in order to reduce machining time and measurement time. In consideration of safety, these apparatuses are mounted with a function for performing an emergency stop even during driving. An emergency stop of an apparatus occurs when an emergency shutdown of the apparatus is performed manually by pressing an emergency stop switch, when the apparatus automatically stops upon entering an alarm condition due to a servo motor overload or the like, when power supply is lost due to a power outage, and the like. A general emergency stop method involves decelerating all axes, applying braking, and stopping while maintaining positions.
However, an emergency stop performed while respective axes of a machine tool are being driven at high speed is not always safe because inertia causes movement over a certain distance until the axes come to a stop due to braking. With a machine tool during machining, a tool and a workpiece may collide with each other, and with a three-dimensional measuring apparatus during measurement, a probe and a measured object may collide with each other. A method has been proposed for maintaining servo motor control until respective axes come to a complete stop in order to safely stop an apparatus even in a case of an emergency stop during such high-speed driving.
FIG. 24 shows a flow chart of an emergency stop process according to conventional art.
A judgment is made on whether or not an emergency stop is caused by a power outage, and if so, supply of power is started from an uninterruptible power source (UPS), a deceleration stop is performed while maintaining control of respective axes, a retraction is performed to a safe position, and braking is applied to all axes. Subsequently, the emergency stop is completed by suspending control of all axes. With this technique, by maintaining servo motor control until the respective axes come to a complete stop, control of the respective axes is suspended midway through an original drive path, the respective axes are then moved to a safe position, and brakes are actuated. When the power source is lost due to a power outage or the like, control of the respective axes is suspended in a safe manner by receiving supply of power for a certain period of time from a separately-installed uninterruptible power source (UPS) and from regenerative energy due to deceleration (refer to Japanese Patent Application Laid-Open No. 8-54914 and Japanese Patent Application Laid-Open No.
In addition, Japanese Patent Application Laid-Open No. 2006-177437 and Japanese Patent Application Laid-Open No. 2006-214536 disclose a technique related to an air balance structure applied to a mechanism for compensating for a self-weight of a movable part that moves on a vertical axis in a precision machine tool or a precision measuring device.
As disclosed in Japanese Patent Application Laid-Open No. 8-54914 and Japanese Patent Application Laid-Open No. 8-227307 described earlier, when performing an emergency stop, control of respective axes of an apparatus such as a machine tool or a three-dimensional measuring apparatus is maintained, and after the respective axes are stopped midway through an original drive path, the axes are moved to a safe position and brakes are actuated.
When the power source has been lost due to a power outage or the like, the respective axes are stopped in a safe manner by receiving supply of power for a certain period of time from a separately-installed uninterruptible power source (UPS) and from regenerative energy due to deceleration. While this method is superior in terms of safety and versatility, an expensive battery and an expensive regenerative power supply circuit are separately required. In addition, retrofitting a regenerative power supply circuit or the like requires significant modifications to a control circuit or a power supply circuit. Furthermore, for example, when a tool collides with a workpiece and an overload alarm occurs due to an erroneous machining program, maintaining an erroneous tool path prior to stoppage may result in further damage. Furthermore, there is also a problem in that a path cannot be maintained if an acceleration exceeding a driving force of each axis is applied from the outside such as during an earthquake.